Such salt cores for casting purposes that are flushed out of parts after casting as well as attempts at optimizing them by admixing additives have been long known. In DE-C-14 83 641 it has been found that adding up to 10% of borax, magnesium oxide or talcum improves the load capacity of salt cores consisting of NaCI and/or KCI. DE-A-19 34 787 proposes to add a synthetic resin binder and water glass in order to avoid pressing and sintering. These admixtures are also known from U.S. Pat. No. 3,764,575.
The use of synthetic resin binders however is very problematic since they are subject to carbonization and outgassing. This is disadvantageous for casting so that the salt cores are usually pre-heated to a temperature of 600° C. in order for outgassing to occur prior to casting. Since the synthetic resin binders for their main part release stressing gases, a suction means is needed for this process. Another problem arises from the manipulation of the very hot salt cores when placing them into the casting mold. From DE 195 25 307 A1 it is known to manufacture a casting core from perlite, sodium hexametaphosphate and water.